It may be a completely different chip.
About 10 months ago Intel released their first consumer dual core chip, the Pentium D. It was pretty much just two Pentium 4 cores on the same piece of ceramic and consumed a ton of power (~130W). Many of the Windows/PC OEMs shipped it, but Apple hadn't switched yet.
Core Duo is the second generation of consumer dual core chips from Intel. It's based on the Pentium M, has much more integration between the cores, and consumes much less power (~30W). Many Windows/PC OEMs ship it in addition to Apple.